1. Field of the Invention
The present invention relates to modified polytetrafluoroethylene (hereinafter referred to as "PTFE") fine powder and production of the same. More particularly, it relates to PTFE fine powder having good paste extrudability and thermal stability and a process for producing such modified PTFE fine powder.
2. Description of the Prior Arts
PTFE can be modified while maintaining its inherent non-melt moldability by copolymerizing tetrafluoroethylene (hereinafter referred to as "TFE") with at least one other fluorine-containing monomer in an amount of not larger than about 2% by weight. Such a copolymer is known as a "modified PTFE" and is distinguished from a melt moldable TFE copolymer. In the present specification, the term "modified PTFE" is used to define PTFE modified with a small amount of a comonomer as described above, although PTFE which is prepared by controlling its molecular weight with a molecular weight modifier such as methane and ethanol, it may still be referred to as "modified PTFE" in a broad sense. Modified PTFE often has physical properties and/or moldability which are not found in "pure" PTFE. The comonomer used for modifying PTFE includes CF.sub.3 -- CF.dbd.CF.sub.2, C.sub.3 F.sub.7 OCF.dbd.CF.sub.2, ClCF.dbd.CF.sub.2, C.sub.4 F.sub.9 CH.dbd.CH.sub.2, and mixtures therof.
The PTFE and modified PTFE are classified roughly into two types. One is a granular resin which is prepared by finely grounding particulate PTFE resin obtained by suspension polymerization and the other is fine powder which is prepared by coagulating the resin from a latex obtained by an aqueous dispersion polymerization (emulsion polymerization) and drying the coagulated resin. The granular resin and the fine powder of PTFE or modified PTFE are practically produced and used, but they are molded in different molding manners. The granular resin is molded by press molding and ram extrusion, and the fine powder is molded by paste extrusion and calendering in which a lubricant is mixed with the fine powder.
Generally, the suspension polymerization is described, for example, in Japanese Patent Publication Nos. 25398/1976 and 31524/1984, and the aqueous dispersion polymerization is described in U.S. Pat. No. 2,965,595. Both polymerization methods are carried out in deionized deoxygenated water contained in a polymerization reactor equipped with a temperature controller and a stirrer by adding various additives in the water, replacing the interior of a reactor several times with nitrogen gas, adding TFE under pressure and then charging the modifier comonomer and an initiator to copolymerize TFE and the modifier.
As the additives, there are exemplified a buffer, a molecular weight modifier, an initiating aid, a non-tackifier, a fluorine-containing dispersant (surfactant) and the like. The main difference between the suspension polymerization and the aqueous dispersion polymerization resides in that the former uses no or little dispersant while the latter uses the dispersant in a sufficient amount for stably dispersing latex particles in an aqueous medium, for example, in an amount of about 100 to 10,000 ppm.
The present invention generally relates to the find powder type modified PTFE resin. As described above, this type of modified PTFE resin is used mainly in paste extrusion in which the resin is mixed with a liquid lubricant and extruded to form a comparatively thin rod or tube. The extrudate is rolled to form a film and used as a so-called raw tape or sintered in the case of a tube or a wire coating.
From an industrial view point, it is required for the PTFE fine powder to be paste extruded with efficiency under high productivity, and a sintered extrudate should have large strength. To achieve high productivity, a reduction ratio (RR) during extrusion is important. The reduction ratio is a ratio of a cross section (S) of a cylinder filled with the fined powder to be extruded to a cross section (s) of an outlet of an extruder die (S/s). The larger reduction ratio is preferred to charge a larger amount of the PTFE fine powder in the cylinder and extrude it in one extrusion cycle. The reduction ratio is measured and compared by extruding the PTFE fine powder under the same extrusion condition such as an extrusion ratio. The possible upper limit of the reduction ratio varies with the type of PTFE fine powder. When the reduction ratio exceeds the upper limit attributed to a specific PTFE fine powder, any normal extrudate cannot be produced. For example, the extrudate is extruded in a zig zag line or in a spiral form, a surface or an inner part of the extrudate is cracked by shear force, or the extrudate is broken during extrusion. To improve the paste extrudability under high reduction ratio (hereinafter referred to as "HRR extrudability"), many techniques have been proposed.
One of the most early proposals is a TFE polymerization method disclosed in U.S. Pat. No. 3,142,665 and its corresponding Japanese Patent Publication No. 4643/1962. This TFE polymerization method is characterized in that the modifier is introduced to a polymerization system before 70% of TFE is consumed and polymerized. As the modifier, not only copolymerizable monomers are exemplified but also methanol and the like. The introduction of the modifier to PTFE reduces the crystallinity of the polymer and the suppresses and inherent fibrillation property of the PTFE. As the reduction ratio increases, the extrusion pressure rises. However, the extrusion at high pressure applies large shear force on the primary particles and tends to enhance fibrillation of the particles, which results in an abnormal flow of the resin during extrusion and formation of defects or cracks in the extrudate or the sintered article. Therefore, the suppression of the fibrillation property of PTFE by the introduction of the modifier reduces extrusion defect under HRR due to excess fibrillation of the resin.
The method described in Example 24 of U.S. Pat. No. 3,142,665 comprised adding the modifier to the polymerization system when 60% of TFE to be polymerized was consumed. However, in other Examples of said U.S. Patent, the modifier was introduced from very beginning of the polymerization. Japanese Patent Publication No. 26242/1981 describes the polymerization of TFE using the modifier. In this polymerization, chlorotrifluoroethylene (hereinafter referred to as "CTFE") was introduced when 70 to 85% of the polymerization completed so as to modify a shell part of each resin particle near the particle surface, whereby modified PTFE fine powder which has low extrusion pressure and good HRR extrudability can be obtained.
In addition, it was proposed to improve HRR extrudability of the PTFE fine powder by modifying a core part of each resin particle more than a shell part (cf. Japanese Patent Publication No. 50814/1982).
Further, it was proposed to form a three layered PTFE resin particle in which the first and third layers were modified while the second layer was not (cf. Japanese Patent Publication No. 92943/1981).
The introduction of the modifier to the PTFE fine powder may improve some properties of the sintered final product rather than the paste extrudability of the PTFE fine powder. Sometimes, the modifier is used to this end. Particularly when the copolymerizable modifier is used, transparency of the sintered article of PTFE is improved, and sintering time can be shortened. For example, U.S. Pat. No. 3,819,594 describes that when perfluroalkyl vinyl ether is used as a modifier and a persulfate is used as the polymerization initiator, modified PTFE obtained has significantly improved mechanical properties, particularly flexural fatigue strength.
The PTFE fine powder for HRR extrusion is processed in the form of a wire coating or a thin tube and increasingly used in a field wherein better properties are required, for example, in air plane, automobile and precision machine industries. The modified PTFE fine powder, however, may have less thermal stability than the pure PTFE fine powder, particularly when a certain type of the modifier is used. For some types of such modified PTFE fine powder, it is desired to improve thermal stability without deteriorating the HRR extrudability. Although the CTFE-modified PTFE fine powder (cf. Japanese Patent Publication No. 26242/1981) has superior HRR extrudability to other modified PTFE fine powder, it tends to have a larger thermal instability index (TII) as defined by ASTM D-1457 81a. The TII is an index calculated from difference of densities of the articles which are sintered for various periods of time. The smaller TII is preferred, although the large TII does not necessarily mean easy heat deterioration of the PTFE fine powder, or the stress crack found in the wire coating may be caused by the extrusion technique in many cases. For example, measurement of the TII of commercially available PTFE fine powder revealed that the TII of the CTFE-modified PTFE fine powder is mostly in a range between 30 and 50. On the contrary, unmodified PTFE having 2.22 or less of a standard specific gravity (SSG) as defined below and in turn a sufficiently large molecular weight has the TII of not larger than 30. Although PTFE modified with perfluoroalkyl vinyl ether as disclosed in U.S. Pat. No. 3,819,594 has a TII of nearly zero and is satisfactory, it has insufficient HRR extrudability.